How do you setup disk or bandwidth quota upgrades in WHMCS? i.e. clients have the option of pre-purchasing (billed monthly/quarterly/whatever) upgrade blocks of disk space or transfer, which is added onto the limits of their plan, if they expect that they will sue more than their allocated resources.

Dedicated IPs and the like go in the add-ons section, but what about this?

(NB, I'm looking for pre-paid upgrades, not overage charges at the end of the month)